1. Field of the Invention
This invention relates generally to multi-party telephone lines and, more particularly, to a device for providing selective ringing of a specific party in a multi-party telephone system using polarity encoding.
2. Description of the Prior Art
In a multi-party telephone system, several customers have their phone equipment running off of a single telephone company network tip, ring and ground circuit. It is necessary, in this type of system, to provide a means for the telephone central office to selectively ring only one of the parties. One such means is for the central office to polarize the wiring to the ringers of each of the customer's phones. Each customer's phone is polarized in a different manner so that by polarity encoding the ring signal from the central office, only the intended party's phone would ring while the other party's phones would not ring. The ringer for each phone must be wired differently, polarity-wise, from the other phones in the multi-party system. This requires that the phones be specially wired and provided by the telephone company.
However, in the present deregulated environment, it is mandatory that the customer be able to buy and own his own equipment. The commercially available phones all come with ringers wired across tip and ring only, and hence in one and the same polarity. Thus, there is a need for a device that can be easily connected to the subscriber's phone for selectively verifying the proper polarity from the central office and ringing only the intended party.
One such system is disclosed in U.S. Pat. No. 4,310,723 issued to Svala, disclosing a polarity-selective ringing device for use in a two-party system. Unipolar diodes are connected in the speech and ringer circuits in opposite directions for each of the parties. Sensors provide party identification by sensing the direction of the current flow in the line and thereby provide the correct line polarity for the identified party. The system is limited in that it is only for use in a two-party line system.